Improve some very confusing structure names in astobj2.c

Improve some very confusing structure names in astobj2.c

As pointed out by 'akshayb' on #asterisk-dev, the code here called a list of
bucket entries a 'bucket', and the entries within the bucket were called
'bucket_list'. This made the code very hard to understand without reading
all of it... so I've renamed 'bucket_list' to 'bucket_entry' to clarify the
purpose of the structure.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@263208 65c4cc65-6c06-0410-ace0-fbb531ad65f3